Complex modulation techniques such as Amplitude Modulation (AM) and Phase Modulation (PM) with spread spectrums in a code domain multiplex are widely used in new wireless communication systems. For example, Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) are used in 3G wireless communication systems, and Orthogonal frequency-division multiplexing (OFDM) is used in Long Term Evaluation (LTE) wireless communication systems. These wide band modulated waveforms introduce a design challenge on controlling the Radio Frequency (RF) power of the amplifier. In particular, the high peak to average ratio (PAR) signals drive the amplifier to a non-linear region, and such high PAR signals reach the input power limit of the transistor in the amplifier, which increases the risk to cause the amplifier to burn-out during the operation.
Many Digital Pre-Distortion (DPD) amplifying systems use the crest factor reduction (CFR) technique to reduce the signal's PAR so as to improve the amplifier's non-linearity and increase the amplifier's power efficiency. However, on correcting a DPD loop's linearity for the amplifier's AM/PM distortion, the DPD loop needs to expand RF signal's gain/power to compensate the AM distortion on the amplifier's head room characteristics, and such RF signal's gain/power expanding will drive the peak power of the signal to be close to or even over the amplifier's saturated region.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.